OPTICAL AND RADIOCARBON AGES OF EOLIAN SAND IN THE GREEN RIVER LOWLAND, NORTHWESTERN ILLINOIS

Sand dunes are a topographically distinct landform in the Green River Lowland, northwestern Illinois, and are stabilized by vegetation cover under the current climate regime. Although sediments reworked from either glacial-fluvial outwash sand or more recent floodplains are the most likely source for the eolian sand in the Green River Lowland, the dunes' formation histories are poorly known. In this study, the geochronology of the dunes was investigated by both optically stimulated luminescence (OSL or optical dating) and radiocarbon dating. Preliminary dating results suggest that the major dune construction took place around 17,000-18,000 years ago, when the Laurentide Ice Sheet had mostly retreated from Illinois. Our new age control also suggests that dunes were reactivated at least one time during the Holocene, evidenced by a light-colored non-stratified sand layer, sandwiched by two buried soils, that yielded an OSL age of 2,730 ± 210 years ago. The radiocarbon dates on soil organic matter obtained from the two buried soils are 590 ± 40 cal. yr BP and 3,040 ± 100 cal. yr BP (calendar years before present), agreeing well with the OSL dates for the sand layer. The cause of the late Holocene eolian activity in northwestern Illinois is not clear, although we speculate that a shift to arid climate resulted in removal of vegetation cover that anchored the dune sand, ultimately leading to dune activation. More age estimates and work are needed to reconstruct the dune history in the Green River Lowland.